Carbon-carbon multiple bonding is so well understood that ethene, acetylene, and allene (1,2-propadiene) derivatives are the benchmarks for comparisons with the analogues involving multiple bonding between the heavier main group elements (W. Kutzelnigg, Angew. Chem. Int. Ed. Engl. 23, 272-295 (1984), E. Rivard, P. P. Power, Inorg. Chem. 46, 10047-10064 (2007), P. P. Power Chem. Rev. 99, 3463-3503 (1999)). In line with hybridization theory, allenes A1 (see FIG. 1) have a linear CCC skeleton with orthogonal pairs of substituents (N. Krause, A. S. K. Hashmi, Eds., Modern Allene Chemistry (Wiley-VCH: Weinheim, 2004)). The allene framework is so rigid that even minor deviations from linearity are of note. In a paper from 1995, entitled “A remarkably bent allene. X-ray crystal structure and ab initio calculations”, Weber et al. (E. Weber, W. Seichter, B. Hess, G. Will, H. J. Dasting, J. Phys. Org. Chem. 8, 94-96 (1995)) described compound A2, which is still today the most severely bent acyclic allene, with a CCC bond angle of 170.1°. The authors demonstrated that the non-linearity was due to packing effects in the crystal. To significantly bend an allene, it is necessary to constrain the C═C═C π-system into a ring, but limitations rapidly emerge. Even the low temperature NMR characterization of cyclic allenes is limited to those containing more than seven carbon atoms (M. Christi, in Modern Allene Chemistry, N. Krause, A. S. K. Hashmi, Eds., (Wiley-VCH: Weinheim, 2004), pp 243-357; the only exception is the 1,2,4,6-cycloheptatetraene A3, incarcerated in a molecular container by Warmuth (R. Warmuth, M. A. Marvel, Chem. Eur. J. 7, 1209-1220 (2001). The kinetically protected 1,2-cyclooctadiene A4 (calculated CCC angle: 158°) (J. D. Price, R. P. Johnson, Tetrahedron. Lett. 39, 4679-4682 (1986), and the trisilicon (Y. Pang, S. A. Petrich, V. G. Young Jr., M. S. Gordon, T. J. Barton, J. Am. Chem. Soc. 115, 2534-2536 (1993), T. Shimizu, F. Hojo, W. Ando, J. Am. Chem. Soc. 115, 3111-3115 (1993)) and diphosphorus (M. A. Hofman, U. Bergstrasser, G. J. Reiss, L. Nyulaszi, M. Regitz, Angew. Chem. Int. Ed. 39, 1261-1263 (2000)) containing six-membered rings A5-7 (crystallographically observed CCC angles: 166, 161 and 156°, respectively) are the smallest ring allenes isolated. Note that because of the presence of the heavier main group elements, the allene fragment of A5-7 is not significantly more distorted than in the eight-membered ring A4. Smaller ring allenes A8 are only known as reaction intermediates, in line with calculations that indicate a strong diradical character A8′ (K. J. Daoust, S. M. Hernandez, K. M. Konrad, I. D. Mackie, J. Winstanley, R. P. Johnson, J. Org. Chem. 71, 5708-5714 (2006).
In marked contrast with all-carbon allene fragments (C═C═C), crystallographic (S. Ishida, T. Iwamoto, C. Kabuto, M. Kira, Nature 421, 725-727 (2003), T. Iwamoto, H. Masuda, C. Kabuto, M. Kira, Organometallics 24, 197-199 (2005)) and computational studies (M. Rosa, M. Karni, Y. Apeloig, J. Am. Chem. Soc. 126, 10544-10545 (2004), M. Kosa, M. Karni, Y. Apeloig, J. Chem. Theory Comput. 2, 956-964 (2006), B. Pinter, A. Olasz, K. Petrov, T. Veszpremi, Organometallics 26, 3677-3683 (2007)) have shown that allenes based on heavier group 14 elements (E=E=E; E: Si, Ge) (B1-2) are highly flexible, and exhibit a bent structure (136.5° and 122.6°, respectively) even without being confined to a ring. The striking differences in the geometry between all carbon allenes A and their heavier element congeners B is mainly due to the “first long row anomaly”, as described by Grützmacher (H. Grützmacher, Science 289, 737-738 (2000)). The first long row elements tend to form hybrids from s and p orbitals that lead to the familiar linear, trigonal and tetragonal bonding geometries of carbon. Second long row elements largely avoid hybridization (W. Kutzelnigg, Angew. Chem. Int. Ed. Engl. 23, 272-295 (1984)). Among the consequences, second and higher row elements are generally reluctant to form multiple bonds, and therefore heavier element-heavier element π-bonds are weak (W. Kutzelnigg, Angew. Chem. Int. Ed. Engl. 23, 272-295 (1984), E. Rivard, P. P. Power, Inorg. Chem. 46, 10047-10064 (2007), P. P. Power Chem. Rev. 99, 3463-3503 (1999)).
Making an analogy with the heavier main group element allenes B, we reasoned that weakening the π-bonds of all-carbon allenes A should impart a greater flexibility to the CCC skeleton, and therefore allow for the preparation of stable small ring allenes.